A thermoplastic resin, which is excellent in molding-processability and electrical insulation and moderate in price, is widely applied to coating materials for indoor cables, for home electric appliances, for thin electric wires coating of automobiles and the like, and wallpaper etc. So far, a polyvinyl-chloride-based resin has been heavily used for such purposes.
In an event of fire, however, products (for instance, cables) using the polyvinyl-chloride-based resin emit smoke in such an enormous quantity as to impede evacuation and fire-fighting activities in an enclosed space such as inside of an underground mall, a subway, a vessel or the like, which may lead to a secondary disaster. Accordingly, there has been a demand for a resin material that emit less smoke in an event of fire, and that generates less amount of poisonous gas such as carbon monoxide even when combusted. In recent years, out of concern for environmental issues such as dioxin, use of the polyvinyl-chloride-based resin has been less preferred than before.
Consequently, a non-halogen-based resin such as a polyolefin-based resin is more frequently used in place of the polyvinyl-chloride-based resin. However, since the polyolefin-based resin is more combustible than the polyvinyl-chloride-based resin, there have been attempts to add a magnesium-hydroxide-based compound to the polyolefin resin as a non-halogen-based flame retardant so that a flame-retardant polyolefin-based resin is obtained.
One of suggested magnesium-hydroxide-based flame retardants contains, for example, iron compounds, cobalt compounds, chrome compounds, copper compounds, vanadium compounds and nickel compounds such that the total metallic content thereof is less than 0.01 percent by weight (see, Patent Document 1). By reducing the content of such transitional metals, the resin is made more resistant to thermal degradation when the resin is melt-kneaded with the compounds.
Another suggested magnesium-hydroxide-based flame retardant is based on combined metal hydroxide represented by a chemical formula of Mg1-XM2+X(OH)2 (see, Patent Document 2), where M2+ represents at least one of divalent metal ions selected from a group consisting of Mn2+, Fe2+, Co2+, Ni2+, Cu2+ and Zn2+ while X represents a range of 0.001≦X≦0.005 or 0.7≦X≦0.9. By using a magnesium-hydroxide-based flame retardant based on such a composite, acid resistance of a molded article (e.g., a communication cable) is enhanced.
[Patent Document 1] JP-A-2004-002884
[Patent Document 2] JP-A-05-209084